Wide lane, it is mistaken for middle (9)
I believe the answer is:
waistline
'for middle' is the definition.
(synonyms)
'wide lane it is mistaken' is the wordplay.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'mistaken' is an anagram indicator.
'lane'+'it'+'is'='laneitis'
'laneitis' anagrammed gives 'aistline'.
'w'+'aistline'='WAISTLINE'
(Other definitions for waistline that I've seen before include "measurement" , "It goes round trunk" , "Girth" , "Swine tail (anag.)" .)